The present invention relates generally to an instantaneous frequency measurement receiver with bandwidth improvement through phase shifted sampling of real signals.
The conventional Instantaneous Frequency Measurement (IFM) receiver is a radio frequency (RF) receiver used primarily in electronic warfare (EW). Its basic function is to measure the frequency of pulsed signals radiated from hostile radar. Generally, it may be said that IFM receivers measure the frequencies of incoming RF signals utilizing interferometric techniques by detecting the phase shift magnitudes produced in multiple, calibrated delay lines. For instance, the received RF signal is divided and simultaneously introduced into a non-delayed path and a delay line of known length. Since the phase differences between the delayed and non-delayed receiver paths are functions of the input signal frequency, conversion of the phase difference signals to video provides signals whose amplitudes are related to the phase delay. These video signals typically take the form sin .omega..tau. or cos .omega..tau., where .omega. is the angular frequency of the L- processed input signal. The sin .omega..tau./cos .omega..tau. signals are delivered to the encoding network which makes amplitude comparisons of the signals, determines the numerical value of w, and generates the digital frequency descriptive word.
An IFM receiver has many attractive features necessary for EW applications, such as small size, light weight, wide instantaneous bandwidth, and fine frequency resolution.
In a digital rf receiver, the incident radiation is mixed with a local oscillator signal and down converted to an intermediate frequency (IF). This IF signal is discretely sample and further processing is done using digital techniques. The frequency of the incident radiation may be determined by performing a discrete Fourier transform on the sampled signal. The range of frequencies that can be determined by such methods is limited by aliasing effects that limit the frequency range to f.sub.s /2, the Nyquist frequency, where f.sub.s is the uniform sample frequency. In order to measure frequencies over a wide range, a very high sampling frequency must be used.
United States patents of interest include U.S. Pat. No. 4,336,541, to Tsui, which teaches an IFM receiver that detects the difference of two or more RF signals between the onset of the first RF signal pulse and the completion of the frequency encoding strobe. U.S. Pat. No. 4,505,785 to Tucker teaches a sampling spectrum analyzer wherein an input signal is split and each resulting portion is passed down a tapped delay line with samples being taken at each tap processed by separate arithmetic units. U.S. Pat. No. 4,633,516 to Tsui teaches an IFM receiver with an A/D converter which permits elimination of a phase correlator. U.S. Pat. No. 3,708,746 to Willet teaches a system for analyzing the frequency of an incident signal including a digital filter wherein the center frequency of the incident signal is varied by changing the sampling rate.